Current wireless transceivers often need a large number of oscillators (NCOs) due to requirements of multi-channel-, multi-carrier- and multi-band-specifications. The challenges of multiple required coils and possible unwanted coupling between oscillators and transmit frequencies moved digital carrier synthesis into the focus of state-of-the-art developments in frequency and carrier signal generation. Digital synthesis of a carrier frequency for a carrier signal from a reference frequency based on delay lines is one approach followed. This is done via a selection of various phase-shifted replicas from a limited number of possible phase-shifted replicas of the input signal. Due the limited number of replica a quantization error has to be accepted. This approach may result in periodicities, which are caused by a specific repetitive pattern of selection of a quantized frequency (also called tap selection). Those periodicities then may cause unwanted peaks (spurs) in a noise spectrum.
Methods using dithering are applying noise shapers optimized for quantization noise, and a processing of non-linearity in tap lines may not be capable of reducing the spurs sufficiently to meet current requirements for low-noise applications like RX-systems with 100 dBc carrier-to-noise requirements. Conventional solutions may improve spur-levels to −50 or −60 dBc, but this is still far away from the desired −100 dBc spur-levels that may for example be required for LTE-RX.
It is hence desirable to improve a concept for mitigating effects of noise-related spurs arising from a digital synthesis of a carrier signal.